1. Field of the Invention
This invention relates to slurry polymerization of maleic anhydride and (meth)acrylic acid and optionally, with a C.sub.1 -C.sub.4 alkyl vinyl ether, and, more particularly, to a cosolvent system for such polymerization in which a pumpable slurry is formed, from which uniform, fine powders of such copolymers can be obtained within a predetermined molecular weight range and with substantially no residual maleic anhydride.
2. Description of the Prior Art
Copolymerization of maleic anhydride and acrylic acid by precipitation polymerization in a solvent is well known in the art. For example, Walinsky, in U.S. Pat. No. 4,390,670, describes the preparation of substantially homogeneous acrylate/maleate copolymers having a number average molecular weight of 500 to 5000 by copolymerizing 35 to 65 mole percent acrylic acid and 65 to 35 mole percent maleic anhydride in the presence of a polymerization initiator and a chain-transfer solvent selected from a ketone, ester or ether. The ketones disclosed were methyl isobutyl ketone, n-pentanone and n-hexanone; the esters were n-butyl acetate, diethyl phthalate and methyl benzoate; and the ethers were 1,2-diethoxyethane, 1,4-dioxane and diisopropyl ether. The especially preferred solvent was isobutyl ketone. Substitution of xylene or toluene for isobutyl ketone produced a rubbery mass which seriously impeded stirring and resulted in a non-homogeneous copolymer product.
Denzinger, in European Patent No. 103,254 B1, utilized a protective colloid in combination with an aromatic hydrocarbon or aliphatic halohydrocarbon solvent to prevent formation of such aggregates during copolymerization of maleic anhydride and acrylic acid.
Denzinger, in DE 3233775 A1, described a process for making copolymers of maleic anhydride and acrylic acid in a solvent selected from an aliphatic or cycloaliphatic hydrocarbon having from 5-12 carbon atoms. In this case a protective colloid also was used to avoid aggregation.
Wulfvon Bonin, in U.S. Pat. No. 4,390,672, indicated that copolymerization of acrylic acid and maleic anhydride in a solvent was difficult because the solvent tended to reduce the velocity of polymerization and the quantity of maleic anhydride built into the polymer. Accordingly, a solvent-free process was described using a melt of maleic anhydride and an enol ether as a polymerization regulator.
However, these and other processes have not been entirely successful in providing copolymers of maleic anhydride and acrylic acid as (1) a pumpable slurry of the copolymer in a non-toxic solvent; (2) without requiring an excessive degree of agitation; (3) from which predetermined high molecular weight copolymers can be readily obtained; (4) in the form of uniform, fine white powders; (5) having substantially no residual monomers; (6) in a process that does not require a protective colloid; and (7) wherein the solvent can be easily recycled.
Accordingly, an objective of the invention is to provide a process for making copolymers of maleic anhydride and acrylic acid, optionally with other comonomers, which can satisfy the desired features and conditions listed above.
These and other objects and features of the invention will be made apparent from the following description of the invention.